Circular knitting machines are known to one or more needle cylinders which, as shown in FIG. 1, comprise tricks in their outer cylindrical surface. The tricks represent the guides for the needles which during their travel form the stitch loops in cooperation with the sinkers.
The number of tricks is equal to the number of needles which slide reciprocatingly within the tricks. Generally, in machines for producing women's stockings the number of tricks and needles is between 200 and 400 per cylinder. The needles reciprocate between a maximum position and a minimum position into which they are moved by suitable cams acting on the needle and the jack butts.
The cylinder is rotated as are the needles which during their reciprocating movement are fed with yarn in an angularly fixed position when in their highest point of travel. To produce hosiery articles generally only part of the available needles are used at the same time and in the same manner, except for the plain knitwork parts, for which all the needles are operated between their maximum and minimum level. In this mode of operation all of the needles are fed with yarn at each knitting course, and all are moved in the same manner.
When the machine is not producing plain knitwork, in order to produce other types of knitwork (such as mesh or patterned knitwork), some needles are required to produce stitch loops while others have to be raised to an intermediate level to take up yarn without clearing the previous stitch. This allows the formation of a tuck stitch, or have to be raised with a certain delay so that they do not pick up the yarn fed into a certain angular position and therefore do not form new loops. In other words a needle selection has to be made. This means that for each feed it has to be determined which and how many of needles must undergo a certain travel and which and how many other needles must undergo a certain different travel or no travle at all.
This selection is made by the jacks which slide in the same tricks as the needles lying above them, to move the needles to a higher level in order to seize the yarn.
The needles are driven reciprocatingly by fixed cams and counter-cams, which cause them to descend to form the stitch loops.
When the jacks have moved the needle into its working position they withdraw from the needle butt and return downwards.
If the needle, after completing its task of seizing the yarn and forming the stitch loop and therefore being at its minimum level, is not required to pick up a further yarn from another feed, it remains at this level because its control jack remains in its lower rest position.
The jack has a special shape (e.g. slightly curved or bowed) which corresponds to a precise function.
This curvature keeps the jack lightly forced towards the inside of the trick. This ensures the accurate positioning and lack of vibration of the jack by keeping it properly forced against to the trick walls, by requiring the application of a certain force to move it either axially or radially.
The shank of the jack comprises in its middle part an upper guide butt, which comes into engagement with its own control cam for urging the jack downwards when it has completed its task of pushing the needle.
Proceeding downwards along the jack shank there is an intermediate butt which comes into engagement with the lifting cam which raises the jack together with its overlying needle to seize the yarn.
The lowering cam and raising cam are obviously offset angularly and operate at different times on each jack. The foot of the jack comprises the lower guide butt. The lower said butt 8 is known to cooperate with other radial fixed cams which position the butt radially by urging the jack outwards so that its intermediate butt engages the raising cam, which moves the jack vertically upwards.
All the jacks are urged outwards by the radial cams so that their intermediate butt is engaged by the raising cam. The jacks are then raised to urge their needle into its operating position.
This rocking of the jack between its inner and outer position occurs by virtue of its rotation about a pivotal center in its upper part.
The purpose of the selection mechanism is to exclude the jacks which control those needles which, in forming the particular stitch, are not required to be raised.
A known mechanism for selecting or inactivating the needles consists of a plurality of levers or slides which come into contact with a plurality selection of butts on the lower part of the jack, in an intermediate position between the upper butt and the intermediate butt and which urge the jack back into the trick to prevent it from making contact with the raising cam.
The traditional selection procedure therefore consists of bringing a certain number of slides or levers into contact with a certain number of jacks via the selection butts located at the same height, by radially moving only some of the slides towards the outer surface of the cylinder. If a jack is to be left engaged when one or more of the slides have approached the needle cylinder, the selection butts corresponding to the height of those levers are removed from the jack. The number of levers or slides available for selection control is generally equal to the number of available selection butts.
The known selection procedures generally consist of producing contact between the non-removed selection butts of the jacks and the inactivating members, whether levers or slides, by rotating said inactivating members into a position of approach to the cylinder.
Obviously, those inactivating members which are not required to inactivate the jacks and whose butts are in a position corresponding to them are kept in the retracted position at the moment in which they would have made contact.
The devices which operate in accordance with this procedure include those of GB patent Application 2,147,015 A of Bentley Eng. Co. and Italian patents 1,183,228 and 1,186,475 of Officine Savio S.p.A. Needle selection by mechanical devices places very restrictive limits on the machine speed and the possible sequence combinations of needles in their raised position and needles in their lowered position.
The most recently proposed solutions are based on electromagnetic selection of the jacks via a single selection butt, using fixed electromagnetic selection members. Patent publications such as European patent application 219029 in the name of Lonati S.p.A., GB patent application 2,008,157 in the name of Shima Center Co. Ltd, GB patent 1,436,607 in the name of Precision Fukuhara Works Co. Ltd. and French patent 1,564,603 in the name of Mayer & Cie disclose selector devices which consist of a pack of electromagnetically operated selectors positioned at a point preceding each feed.
In GB patent application 2,043,712 in the name of Dainippon and others, the electromagnetic selector device is provided needle by needle, but the technical solutions involved are very complicated, especially for machines of high rotational speed.
These needle selection devices are fixed and operate on the jacks which raise the needles into activation when said jacks, during their rotation together with the cylinder, appear in front of the fixed selection station which precedes each machine feed station.
The time available for setting, initiating and completing the selection is very small, being of the order of a few thousandths of a second. The time is determined by the small angular sector within which the rotating jacks face the selection member, which must be immediately ready to select those needle jacks which at that moment are presented to them.
Most recently, the solution to the problem has turned towards mobile selection devices rotating together with the cylinder, so that the time available for selection is not limited to the moment in which the jacks appear before the stationary selection device. In this manner each jack is constantly presented to its selection member, so that the selection can take place within a wide angle of the cylinder rotation. In this manner the selection setting time is not so drastically small and the selection can be effected reliably and safely.
In U.S. patent application Ser. No. 461,383, needle selection is effected by controlling the radial position of the jacks by means of other corresponding horizontal jacks which slide radially. These horizontal jacks are selected by electromagnetic devices, by assuming a position withdrawn from the cylinder to thus allow the corresponding needle to operate, or a position close to the cylinder to thus inactivate the needle.